The continuity sensor is designed to operate in conjunction with an electrical system consisting of a condition/fault detector and its associated fault indicator. The condition/fault detector is a device which provides an open circuit for normal operation and a closed circuit for a fault condition. The indicator (i.e. warning light, flag, etc.) will turn on in the fault mode as a result of the closed circuit provided by the detector.
The detector is normally mounted remotely with respect to the indicator. The interconnection between them consists of two wires or one wire and a common ground. In aircraft applications, the interconnections typically go through several harnesses, junction boxes, terminal boards, etc.
A typical condition detector in an aircraft application are door closure sensors. They provide an open circuit when the door is properly closed, and a closed circuit when the door is open.
A typical fault detector in an aircraft application are chip detectors used to monitor the health or airworthiness of the engines and gearboxes. Chip detectors employ two conductive zones which are tied to the input leads. The metal chips circulating in the oil system (for engines) or the transmission (for gearboxes) are attracted to the conductive zones by a magnet. An accumulation of the metal chips provides a conductive path across the input leads to complete the circuit.
The operation of the condition/fault detection system depends on the integrity of the interconnections to complete the circuit between the detector and its associated indicator. Should continuity be lost, the detector will not be able to turn the indicator on during the fault mode. Thus, the system has a dormant failure mode wherein the failure of the interconnections gives the same indication as normal operation.
In the prior art, a continuity check of a condition/fault detector system involves two people. In an aircraft installed system, one person sits in the cockpit to close the switch, while the other person physically removes the fault detector from the engine compartment to expose the normally open conductors and shorts across those conductors to complete the circuit. Such a test is impossible to perform when the aircraft is flying. Furthermore, it is undesirable to frequently remove and replace the fault detector for circuit continuity checks as this could lead to error in reinstallation or to damage of the fault detector. What is needed is a device that allows a check of the continuity of the conductors from the cockpit. The device should be simple to connect to an aircraft, requiring no extra conductors.
There is another problem in checking circuit continuity in some condition/fault detection systems. Although an indicator is usually connected to a single condition/fault detector, occasionally an indicator is connected to two or more condition/fault detectors, which are connected in parallel across the indicator. It is desirable to check from the cockpit the continuity of all branches of the circuit, with each branch being checked independently of the other branches.